The characterization of supersensitivity of the cardiovascular system of animals will be pursued. This will be done by a study of the phenomenon after administration of antihypertensive drug such as reserpine and guanethidine, and the drug 6-hydroxy-dopamine. In addition, a study of the desensitization of cardiovascular receptors after high doses of sympathomimetic amines will be done. The role of calcium and other cations and the relationship between supersensitivity and desensitivity will be studied.